Scandium is extremely valuable as an additive for high-strength alloys and an electrode material for fuel cells. However, scandium has not yet been used widely due to the small production quantity and high cost thereof.
Incidentally, a trace amount of scandium is contained in nickel oxide ore such as laterite ore and limonite ore. However, nickel oxide ore has not been industrially utilized as a nickel raw material for a long time since it contains nickel at a low grade. Hence, it has been rarely studied to industrially recover scandium from nickel oxide ore.
Nonetheless, in recent years, the HPAL process has been emerging as a practical method, in which nickel oxide ore is introduced into a pressure vessel along with sulfuric acid, and heated at a high temperature of 240° C. to 260° C. to allow solid-liquid separation into a nickel-containing leachate and a leach residue. In the HPAL process, a neutralizing agent is added to the leachate obtained to separate impurities, and then a sulfurizing agent is added to the resulting leachate from which impurities are separated out, allowing recovery of nickel as nickel sulfide. Subsequently, this nickel sulfide may be subjected to a known nickel refinement process to obtain electrolytic nickel and nickel salt compounds.
In the HPAL processes as described above, scandium contained in nickel oxide ore will be contained in a leachate along with nickel (see Patent Document 1). Subsequently, when a neutralizing agent is added to a leachate obtained from the HPAL process to separate impurities, and a sulfurizing agent is then added, nickel is recovered as nickel sulfide while scandium remains in the acidic solution after addition of the sulfurizing agent. In this way, nickel can effectively be separated from scandium by using the HPAL process.
A method of separating scandium by using a chelating resin has also been documented (see Patent Document 2). Specifically, according to the method disclosed in Patent Document 2, nickel-containing oxide ore is first treated at high temperature and high pressure under an oxidizing atmosphere to selectively extract nickel and scandium into an acidic aqueous solution. Subsequently, the pH of the resulting acidic solution is adjusted to the range of 2 to 4, and nickel is then selectively precipitated and recovered as a sulfide by means of a sulfurizing agent. Next, scandium is adsorbed to a chelating resin by bringing the solution obtained after nickel recovery into contact with the chelating resin, the chelating resin is washed with a dilute acid, and then scandium is eluted from the chelating resin by bringing the chelating resin after washing into contact with a strong acid.
Further, as a method of recovering scandium from the aforementioned acidic solution, the method of recovering scandium by means of solvent extraction has also been proposed (see Patent Documents 3 and 4). Specifically, according to the method disclosed in Patent Document 3, an organic solvent is first added to an aqueous-phase scandium-containing solution to extract a scandium component into the organic solvent, the organic solvent comprising 2-ethylhexyl sulfonic acid-mono-2-ethylhexyl diluted with kerosene, and the aqueous-phase scandium-containing solution containing one or more of at least iron, aluminum, calcium, yttrium, manganese, chromium, and magnesium in addition to scandium. Then, in order to separate yttrium, iron, manganese, chromium, magnesium, aluminum, and calcium extracted into the organic solvent along with scandium, an aqueous solution of hydrochloric acid is added, and scrubbing is performed to remove these elements. Then, an aqueous solution of NaOH is added to the organic solvent to transform scandium remaining in the organic solvent into a slurry containing Sc(OH)3, and the slurry is filtered to obtain Sc(OH)3, which is then dissolved in hydrochloric acid to obtain an aqueous solution of scandium chloride. Then, oxalic acid is added to the aqueous solution of scandium chloride thus obtained to obtain a precipitate of scandium oxalate, and this precipitate is filtered to separate iron, manganese, chromium, magnesium, aluminum, and calcium into the filtrate and then calcined to obtain high purity scandium oxide.
Moreover, Patent Document 4 describes a method of selectively separating and recovering scandium from a scandium-containing supply liquid, the method comprising: bringing the scandium-containing supply liquid into contact with an extracting agent at a certain ratio in a batch process.
As the grade of scandium recovered by these methods, it is known that a purity of about 95% to 98% in terms of scandium oxide is obtained. However, a higher purity, for example, a grade of about 99.9% is required in order to exert favorable properties in applications such as electrolytes of fuel cells of which the demand has increased in recent years although it is a sufficient grade for applications such as addition to alloys.
However, various impurity elements such as manganese and magnesium are contained in the nickel oxide ore described above in addition to iron and aluminum although the kinds and amounts thereof vary depending on the region from which the nickel oxide ore is mined.
The impurity elements have acceptable upper limits of grade and each element is required to be separated and removed to a content equal to or less than the acceptable limit in the case of using scandium in applications such as electrolytes of fuel cells.
However, some impurity elements exhibit behavior similarly to that of scandium in the chelating resins and the organic solvents disclosed in Patent Document 2 and Patent Document 3, and it is thus difficult to effectively separate and recover scandium. In addition, impurities such as iron and aluminum are contained in the leachate of nickel oxide ore at much higher concentrations than scandium, and a method suitable for industrial recovery of high purity scandium from nickel oxide ore has not been found out since the recovery of scandium is also affected by these large amounts of impurities.
As described above, it has been difficult to efficiently recover high purity scandium by effectively separating a large variety of impurities such as iron and aluminum contained in large amounts even when it is attempted to recover scandium from nickel oxide ore.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. H03-173725    Patent Document 2: Japanese Unexamined Patent Application, Publication No. H09-194211    Patent Document 3: Japanese Unexamined Patent Application, Publication No. H09-291320    Patent Document 4: PCT International Publication No. WO2014/110216